1. Field of the Invention
The present invention relates to injection molding apparatus and parts therefore. More particularly the invention relates to an improved apparatus having an injection or shut off nozzle which is greatly simplified in construction and uses a minimum of moving parts.
2. Description of the Prior Art.
Gas-assisted injection molding is known in the art as a preferred way of making larger and more complicated parts without having the problems of sink marks and the like in thick rib sections or when complicated cross-sections are involved. In gas-assisted injection molding these large and complicated parts are made of a hollow construction by injecting a fluid or gas into the interior of a part while it is being formed in a mold cavity. It is known that the gas or fluid which is injected into the interior of the mold cavity will follow the path of least resistance. Since it is well known that the molten plastic will generally cool and harden from the outside inwardly, the gas will penetrate the inner, softer, and warmer portions of the part, which would normally be found in the thicker sections where there is a structural rib, etc. By following this path there is created a hollow within the plastic, and that hollow could be expanded by the addition of pressure outwardly to fill the mold cavity. It is known that if the gas pressure is held at an effectively constant level while the plastic part cools, the surface finish of the plastic part will be greatly enhanced by the minimization or elimination of sink marks, etc.
However, along with the idea of the injection of the gas or fluid came the need to vent the gas to relieve the pressure within the part before the mold could be opened. Many solutions were tried to discover a satisfactory way of introducing the gas into the interior of the plastic part, and then to vent the gas before opening the mold cavity.
One such apparatus for venting the gas before opening the mold cavity is described in U.S. Pat. No. 4,943,407 issued Jul. 24, 1990 to James W. Hendry. This is the closest prior art of which Applicant is aware. Generally this invention involves venting the gas to atmosphere through a specialized sprue bushing in which the sprue bushing includes a first body part in which a second body part or cylindrical pin is mounted for movement upon the activation of an actuator. The pin has two flow paths which are alternately used. One path allows the passage of plastic through the sprue bushing into the sprue. The second path allows injection or venting of gas from the interior of the associated mold cavity.
However, mold sprues are generally small, and the moving parts thereof which allow the injection of plastic and gas alternately have proven to be difficult to deal with. In addition, the providing of these special sprue valves adds significantly to the cost of the molds.
Thus, Applicant's assignee, to eliminate the problems in the prior art, developed the idea of venting through the injection nozzle, which is a larger device, can be more easily manufactured, and can serve multiple molds. Such a nozzle is described in co-pending U.S. Letters Patent Application Ser. No. 07/628,746, filed on Dec. 17, 1990, now U.S. Pat. No. 5,200,127 and assigned to the common assignee of the present invention.